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Auto paintwork problem: volcanoes forming under the paint

I am doing some digging into the corrosion of auto panels with particular reference to lacquer finished, multi-coat painted, pre-treated panels - e.g door panels. I am interested to know the cause of small "volcano" shaped corrosion points which appear to come from behind the paint surface and "erupt" to leave a small weeping rust point. Are these caused by deficient pre-treatment or perhaps moisture being left on the panel surface before painting which then becomes trapped behind the paint/lacquer finish which is impermeable etc etc? Can anyone help me with experiences, comment or further references?

Many thanks,

John M Barton
consultantcy - Buxton, Derbyshire, UK
2003

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Hello John. I'm trying to help you, not harass you, but when you say you are "doing some digging into" a problem, you are not being clear enough for us to help you. We don't know if there is a rust spot on the door panels of a used car you bought, or whether you are leading a major corrosion study commissioned by an OEM. Step 1 in the first case might be to buy a scrap of sandpaper whereas step 1 in the second case might be to buy a scanning electron microscope :-)

Please get back to us to clear up our question. Thanks!

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
2003

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Ok point taken - I'm advising a group of guys who have a similar rusting problems on their autos and are contesting the refusal of their warranty claims. These are particular to individual panels (doors for example) which would be pre-treated as a separate item to the main body shell.

[returning]
consultancy - Buxton, Derbyshire, UK
2003

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Hi, John. My experience may not be universal enough or conclusive enough to settle the issue, but when I see volcanoes, it's usually indicative of corrosion of materials that create voluminous corrosion products, like zinc does, rather than steel. So I'd guess that this area of these panels is galvanized. Yes, I think it's possible that the doors were not dried, and were allowed to try to air dry while stacked close together, such that what you are seeing is "wet storage stain" underneath the paint.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
September 19, 2009

Hi. I have the exact same problem on an 04 Ford truck. There was a large spot of paint blistering. I sanded these off and sanded down to shiny metal and applied primer. In two days there were tiny eruptions of rust colored fluid directly from the metal and through the primer. It's really strange. If anyone can help I'd appreciate it...Greg

greg almost
- vanceburg, Kentucky
March 15, 2010

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Hi, Greg. I don't know where the moisture could be coming from except the outside world. I think the moisture permeates through the primer to try to wet the hydroscopic metal, rather than coming from within solid metal. I believe the answer is to apply rust converter ⇦ this on eBay or Amazon [affil links] before the primer. This is a phosphoric acid based product that converts red rust to the more stable black rust. If the panels were galvanized (zinc coated), the phosphoric acid will also etch the surface for better adhesion. Good luck.

Regards,

Ted Mooney, P.E.
Striving to live Aloha
finishing.com - Pine Beach, New Jersey
March 16, 2010